The invention generally relates to a method and apparatus for collecting condensate from process streams in an integrated fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and 
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell. 
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and orifices to, as examples, route the reactants and products through the fuel cell stack. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time. Fuel cell systems adapted to accommodate variable loads are sometimes referred to as xe2x80x9cload followingxe2x80x9d systems.
Fuel cells generally operate at temperatures much higher than ambient (e.g., 50-80xc2x0 C. or 120-180xc2x0 C.), and the fuel and air streams circulated through the fuel cells typically include water vapor. For example, reactants associated with sulphonated fluorocarbon polymer membranes must generally be humidified to ensure the membranes remain moist during operation. In such a system, water may condense out of a process stream where the stream is cooled below its dew point. For example, if the anode and cathode exhaust streams are saturated with water vapor at the stack operating temperature, water will tend to condense from these streams as they cool after leaving the stack. Similarly, the humidity and temperature conditions of other process streams may also produce condensation. It may be desirable to remove condensate from a process stream in a fuel cell system process stream. As examples, such condensate can interfere with the flow of process streams, can potentially build to levels that can flood portions of the system, and can also cause problems if allowed to freeze (e.g., in an outdoor unit that is not in service).
The term xe2x80x9cintegrated fuel cell systemxe2x80x9d (also commonly referred to simply as xe2x80x9cfuel cell systemxe2x80x9d) generally refers to a fuel cell stack that is coupled to components and subsystems that support the operation of the stack. For example, this could refer to a fuel cell stack that is connected to a power conditioning device that converts direct current from the fuel cell into alternating current similar to that available from the grid. It might also refer to a system equipped with a fuel processor to convert a hydrocarbon (e.g., natural gas, propane, methanol, etc.) into a hydrogen rich stream (e.g., reformate) for use in the fuel cell. An integrated fuel cell system may also include a control mechanism to automate at least some portion of the operation of the system. Integrated fuel cell systems may include a single controller common to the entire system, or may include multiple controllers specific to various parts of the system. Likewise, the operation of integrated fuel cell systems may be fully or partially automated. Also, an integrated fuel cell system may or may not be housed in a common enclosure.
There is a continuing need for integrated fuel cell systems and associated process methods designed to achieve objectives including the forgoing in a robust, cost-effective manner.
The invention generally relates to a method and apparatus for collecting condensate from process streams in an integrated fuel cell system. In one aspect, the invention provides a water management subsystem for a fuel cell system. A gas conduit contains a gas at a first pressure (e.g., a fuel cell system process stream such as a cathode or anode reactant stream). A water tank in the system contains water at a certain level. The terms water tank and water collection tank are used interchangeably in this context, and generally refer to any vessel adapted to accumulate water in the system. The water tank has an inlet orifice below the water level. A drain conduit has a first end and a second end. The drain conduit is connected at the first end to the gas conduit, and the drain conduit is connected at the second end to the inlet orifice of the water tank. The water level and the inlet orifice have a vertical height of water between them corresponding to a head pressure greater than the first pressure. In this context, it will be appreciated that head pressure refers to the pressure exerted by a vertical column of water.
Various embodiments of the invention can include additional features, either alone or in combination. For example, the system can further include a water level sensor adapted to measure the water level. The water tank can have a second inlet orifice, and have a water supply (e.g., a municipal water line) connected to the second inlet orifice. A controller can be connected to the water level sensor, being adapted to feed water to the tank from the water supply when the sensor indicates the water level is below a predetermined threshold. For example, it may be desirable to keep a level of water in the tank such that the pressure at the inlet orifice leading to the drain conduit is greater than the pressure of the gas in the gas conduit (e.g., to prevent the gas from blowing into the water tank).
In some embodiments, a water level sensor is provided to measure the water level. The water tank has a drain (e.g., to the sewer), and a controller is connected to the water level sensor, such that the drain is opened when the sensor indicates the water level is above a predetermined threshold, and the drain is closed when the sensor indicates the water level is below a predetermined threshold.
An examples, the gas conduit can be an anode tailgas oxidizer, or a conduit associated with an anode tailgas oxidizer such as an inlet stream or exhaust stream. The gas conduit can also be an anode fuel outlet conduit of a fuel cell, or an anode fuel inlet conduit of a fuel cell.
In some embodiments, the water tank can include a gas inlet and a gas vent, wherein at least a portion of a cathode inlet air stream of a fuel cell is circulated through the water tank from the gas inlet to the gas vent. As an example, such an arrangement may be desired to continually flush the atmosphere in the water tank of any combustible components that might otherwise accumulate. In some embodiments, a cathode exhaust stream is circulated through the water tank instead. In some embodiments, such a gas vent is in fluid communication with an air inlet of an oxidizer. For example, the air purged from the water tank can be used to provide oxygen to the ATO.
In another aspect, the invention provides a water management subsystem for a fuel cell system that has a gas conduit containing gas at a first pressure. A water collection tank contains water and an atmosphere (i.e., the gas above the water level). The tank has an inlet orifice below the water level in the tank. The atmosphere of the tank has a second pressure. A drain conduit, having a first end and a second end, is connected at the first end to the gas conduit, and is connected at the second end to the inlet orifice of the water collection tank.
The water level and the inlet orifice have a vertical height of water between them corresponding to a head pressure, and the sum of the second pressure and the head pressure is greater than the first pressure. In this arrangement, condensate in the gas conduit is allowed to drain into the water tank through the drain conduit. Since the pressure at the tank inlet orifice is greater than that of the gas conduit, the gas in the gas conduit is not allowed to blow through the water tank.
In another aspect, the invention provides another water management subsystem for a fuel cell system. A gas conduit contains a gas at a first pressure. A water collection tank contains water and an atmosphere, the water having a level within the tank, the water collection tank having an inlet orifice above the water level, and the tank atmosphere having a second pressure. A drain conduit has a first end and a second end, and the drain conduit is connected at the first end to the gas conduit, and is connected at the second end to the inlet orifice of the water collection tank. A portion of the drain conduit forms a water trap bend (e.g., a xe2x80x9cj-trapxe2x80x9d or xe2x80x9cp-trapxe2x80x9d or other similar arrangement). The water trap bend contains water, and has a vertical height corresponding to a head pressure. The sum of the second pressure and the head pressure is greater than the first pressure.
In another aspect, the invention provides a method of water management for a fuel cell system, including at least the following steps: (1) flowing a fuel cell process stream containing liquid water through a gas conduit at a first pressure; (2) draining the liquid water from the gas conduit into a drain conduit; (3) draining the liquid water through the drain conduit into an inlet orifice of a water collection tank, wherein the inlet orifice is located below a water level of the water collection tank; and (4) maintaining the water level of the water collection tank such that a second pressure of water at the inlet orifice is greater than the first pressure of the process stream.
Some embodiments may include additional steps, either alone or in combination. For example, an additional step may include circulating air through the water collection tank, or circulating a cathode exhaust stream from a fuel cell through an atmosphere of the water collection tank to an oxidizer. Embodiments of methods under the invention may also refer to any of the systems and combinations of features described herein.
In another aspect, the invention provides a method of water management for a fuel cell system, including at least the following steps: (1) flowing a fuel cell process stream containing liquid water through a gas conduit at a first pressure; (2) draining the liquid water from the gas conduit into a drain conduit; (3) draining the liquid water through the drain conduit into an inlet orifice of a water collection tank, wherein the inlet orifice is located above a water level of the water collection tank; and (4) maintaining the water level of the water collection tank such that a second pressure of water at the inlet orifice is greater than the first pressure of the process stream.
Advantages and other features of the invention will become apparent from the following description, drawings and claims.